The present invention relates to a respiratory gas concentration measuring apparatus for measuring the concentrations of CO.sub.2, N.sub.2 O, anesthesia gas or other gases in respiratory gas based on the absorption of gases to be measured with respect to light of a particular wavelength which passes therethrough.
One known type of respiratory gas concentration measuring apparatus is shown in Japanese Laid-Open Patent Publication No. Sho 57-23843. According to the disclosed apparatus, which is illustrated in FIG. 1 of the accompanying drawings, a respiratory gas concentration is measured on the basis of the Lambert-Beer's law: EQU Vs=Voe.sup.-KC (1)
where Vs: photoelectric conversion output, Vo: Vs at the time of zero gas concentration, K: proportionality constant, and C: gas concentration.
The prior apparatus as shown includes a connector tube 2 having a tube end 210 to be held in the mouth of an examinee 1, an opposite open end 220 to be vent to atmosphere or connected to an artificial respiratory system, an anesthetizer or the like, and a pair of intermediate windows 3 opening at central portions of the connector tube 2 in confronting relation and rendered airtight by transparent sapphire or the like. The apparatus also has a light source 4, a power supply 5, and a rotatable chopper 6.
The chopper 6 has a pair of filters 7, 8 disposed in diametrically opposite relation. The filter 7 is capable of passing therethrough only light having a wavelength that can be absorbed by CO.sub.2 gas. The filter 8 allows passage therethrough of light having a wavelength that cannot be absorbed by CO.sub.2 gas. The chopper 6 is rotated at a constant cyclic period by a motor 9. A photodetector 10 serves to convert an amount of light falling thereon into a corresponding electric signal, which is fed to an amplifier 11. An output from the amplifier 11 is supplied to a pair of first and second detectors 12, 13. The first detector 12 serves to detect a signal in synchronism with positional alignment of the filter 7 with a path of light (shown by the dotted lines in FIG. 1) passing through the windows 3. The second detector 13 can detect a signal in synchronism with positional alignment of the filter 8 with the light path. A divider circuit 14 effects a division with the output from the second detector 13 as a denominator and the output from the first detector 12 as a numerator. An output from the divider is converted by a logarithmic amplifier 15 into a corresponding logarithmic value which is delivered out as a CO.sub.2 concentration. A ray of light emitted from the light source 4 first falls on a lower one of the windows 3, then passes through a respiratory gas in the connector tube 2 and through the other window 3, and finally reaches the photodetector 10, as indicated by the arrowheads. When the motor 9 is energized, the chopper 6 which is interposed between the window 3 and the photodetector 10 is rotated to interrupt the light path periodically, thereby allowing light to be transmitted intermittently through the chopper 6. Therefore, the photodetector 10 converts the intermittent light into an electric signal. The amplifier 11 amplifies the electrical signal output entering as an input from the photodetector 10.
The first detector 12 detects an output from the amplifier 11 in synchronism with passage of the light through the filter 7 and produces an output which corresponds to Vs in the equation (1). The second detector 13 detects an output from the amplifier 11 in synchronism with passage of the light through the filter 8 and produces an output Vc which is free of any influence of the CO.sub.2 concentration, that is, corresponds to Vo in the equation (1). The output Vo tends to drift due to variations in the amount of light given off from the light source 4, variations in the sensitivity of the photodetector 10, and other factors. On the assumption that Vc=Vo, an output voltage V.sub.D from the divider circuit 14 is expressed by: ##EQU1## Thus, any influence of drifts can be compensated for. However, if temperature-dependent coefficients of detected outputs dependent on the light rays of the different wavelengths (absorbed wavelength and unabsorbed wavelength) are different from each other due to variations in the temperature of the light source or characteristics of the photodetector, then Vc.noteq.Vo, resulting in drifts. The photodetector which uses e.g. PbSe in practice is still subjected to temperature-dependent drifts for the reason described above.